Propagating a high order mode in optical fiber transmission systems can have advantages over propagating a fundamental or basic mode. For example, propagating a high order mode can improve the overall performance of transport systems. One way performance is improved is due to the choice of fiber that will support the propagation of the high order mode. A significant advantage of this high order mode fiber is that it can be designed to have strong negative dispersion and high effective areas. Therefore, it can be used to compensate for chromatic dispersion. The most favorable modes for this application in fiber optics are the even modes (LP.sub.01, LP.sub.02, LP.sub.03 . . . ) because their properties are independent of polarization when being transmitted in a circularly symmetric fiber. There is a drawback to these high order mode fibers, however. Propagating a high order mode can generate interferometric noise due to mode coupling in the fiber. To substantially reduce this noise, it would be advantageous to propagate a single high order mode.
Various methods have been suggested for transforming or coupling light energy in the fiber from one mode to another different mode. For example, a long period fiber grating may be used to transfer energy from one mode to another mode. Unfortunately, this method can also transform some of the light energy from one mode to other undesirable modes. Other methods can have the same undesired results.
Energy transfer or coupling from the desired high order modes to undesirable modes can also occur due to inhomogeniety of the high order mode fiber. These inhomogenieties can occur in the manufacturing process. Inhomogenieties can also appear due to imperfect splicing of the fiber, periodic bending (micro bending), and scattering mechanisms (i.e., Riley scattering), for example.
The result of imperfect mode transformation and mode coupling in the fiber is that undesirable modes will propagate in the fiber. These modes can interfere with the desired mode through a process called multipath interference (MPI). MPI causes significant reduction in signal quality by distorting its phase and amplitude. Therefore, in order to realize reasonable signal quality, the ratio between the energy transmitted through the undesired modes to the energy transmitted through the desired mode should be below 1:10000 or approximately 40 dB. This ratio should be maintained for any length of high order mode fiber being used in order to maintain reasonable signal quality
It is therefore desirable to suppress the undesired modes in a high order mode fiber in order to improve the signal quality in the fiber.